The present invention relates to a communication apparatus used in a digital mobile communication system such as a car/portable telephone system, a cordless telephone system, or a radio LAN system and, more particularly, to a spread spectrum communication apparatus for performing radio communication by using a code division multiple access (CDMA) scheme or a multicode transmission scheme.
Spread spectrum communication, which is resistant to interference and disturbance, has gained a great deal of attention as one of communication schemes used for a mobile communication system.
A spread spectrum communication system performs communication as follows. First of all, the transmitting communication apparatus modulates digital speech data or digital image data by digital modulation such as PSK modulation. The modulated data is converted into a broadband baseband signal by using spreading codes. The spread transmission signal is converted into an RF signal and is then transmitted. The receiving communication apparatus despreads the received RF signal by using the same spreading codes as those used in the transmitting communication apparatus. Digital demodulation is then performed for the despread reception signal by digital demodulation such as PSK demodulation, thereby reconstructing original data from reception data.
In a system of this type, a RAKE receiver is used as one of measures against multipath interference. In a radio communication system, the radio wave transmitted from the transmitting apparatus may directly arrive at the receiving apparatus, or may arrive at the receiving apparatus upon being reflected by a building, a mountain, and the like. If one radio wave reaches the receiving apparatus through a plurality of paths with different delay times, waveform distortion occurs. This phenomenon is called multipath effects.
According to spread spectrum communication, multipath radio signals received by one antenna with different delay times can be separated in units of spreading code length (chip interval). The received multipath signals are respectively input to a plurality of independent demodulators. These demodulators despread the signals with spreading codes of time phases corresponding to the respective paths. The despread reception signals from the respective paths are synthesized into symbols, reconstructing original data from reception data. This reception scheme is called RAKE reception because the receiver has a RAKE-like arrangement. With the use of a RAKE receive, path diversity is obtained. This can greatly improve the reception quality of the signal transmitted through a multipath transmission channel.
In a RAKE receiver, after delay units give different delay amounts to the multipath signals received by a radio circuit for the respective paths, multipliers multiply the resultant signals by the spreading codes generated by a spreading code generator, thereby separating the reception signals for the respective paths. These reception signals are integrated by integral damping filters for a 1-symbol interval. Synchronization and weighting are performed for the integral outputs, and the resultant output signals are synthesized. The above weighting is performed by determining weights in proportion to the amplitudes of the reception signals detected from the respective paths, and multiplying the integral outputs by the determined weights. In this RAKE receiver, therefore, the reception signals from a plurality of paths are subjected to maximum ratio synthesis. As a result, a maximum diversity gain can be obtained.
The following problems to be solved are, however, posed in this conventional RAKE receiver.
Assume that the reception signals from the respective paths are synthesized by maximum ratio synthesis. In this case, when noise superposed on the respective paths can be regarded as equipower white Gaussian noise, a maximum gain can be obtained. For example, in a CDMA mobile communication system, on the upstream channels from mobile stations PS-A, PS-B, and PS-C to a base station BS, waves A2, B1, B2, C1, and C2 become interference waves with respect to a desired wave A1, and the waves A1, B1, B2, C1, and C2 become interference waves with respect to the desired wave A2, as shown in FIG. 6. That is, on upstream channels in the CDMA mobile communication system, interference waves originating from many transmission sources arrive at the base station through various paths. For this reason, the interference waves superposed on all the paths practically become white Gaussian noise owing to the statistical multiplexing effect. If maximum ratio synthesis is performed in this state, a maximum gain can be obtained.
In contrast to this, when orthogonal codes are used as spreading codes on downstream channels from the base station BS to the mobile station PS-A as shown in FIG. 7, the signals propagating through the same path do not become interference waves, and only the signals propagating through other paths become interference waves. That is, as shown in FIG. 8, the waves A2, B2, and C3 become interference waves with respect to the desired wave A1, and the waves A1, B1, and C1 become interference waves with respect to the desired wave A2. For this reason, the interference waves superposed on the respective paths cannot be regarded as equipower white Gaussian noise.
When, therefore, the spread spectrum signal transmitted from one radio station is to be received by a terminal apparatus through a multipath transmission channel, the reception signal output from the RAKE receiver does not have an optimal desired-to-undesired signal ratio (D/U) even if maximum ratio synthesis is performed.
The above problems may arise even in a spread spectrum communication system for performing so-called multicode transmission, i.e., transmitting data from one radio station to another radio station by using a bundle of channels using different spreading codes.